This application is related to Japanese Patent application no. 298965/1992 filed Nov. 9, 1992, incorporated herein by reference in its entirety.
1. Field of the Invention
This invention relates to a novel field-effect transistor (FET). More particularly, this invention relates to an FET composed of oxide layers and, in particular, to an FET that is useful as a non-volatile memory of an integrated circuit.
2. Discussion of the Background
Memories represent a substantial part of the semiconductor market. Besides RAMS, an ever-increasing share is taken by non-volatile memories. In recent years, some elements for non-volatile memories have been manufactured with a capacitor on a gate electrode of a silicon metal-oxide-semiconductor (Si-MOS). The elements utilize metal-nitride-oxide-silicon transistors in which the charges are trapped at the nitride oxide interface. The memory principle is based on Fowler-Nordheim tunneling to move charges from the substrate in the oxide or vice versa. Such memories are called electrically erasable programmable read-only memories (EEPROMS) where the charges trapped in the gate electrode can be removed electrically. EEPROMs are, however, disadvantageous in that they permit relatively slow erase and rewrite operations. Additionally, the number of rewrite cycles is extremely restricted because the rewrite operation requires relatively high voltage and the memories may thus be damaged after a large number of cycles. On the contrary, dynamic random-access memories (DRAMs) have fast access times but conventional Si oxide film systems are unable to provide sufficient level of modulation when DRAMs are integrated to have memory capacities of 256 Mb or more. In this respect, one object of this invention is to provide memory structures using perovskite oxide dielectrics such as SrTiO.sub.3 having a large dielectric constant.
While the condition of charge storage of typical EEPROMs should be controlled externally, application of a unique EEPROM as a memory cell implementing ferroelectrics has also been developed. Ferroelectrics are crystalline substances having a permanent spontaneous electric polarization that can be reversed by an electric field. Thus, ferroelectrics are expected to be used for keeping desired conditions of charge storage without external control. A memory of the type described has long been studied and, in more recent years, active studies and considerations have been undertaken, stimulated and spurred by applications of the above mentioned perovskite dielectric to DRAMs. These memories are called ferroelectric random-access memories (FE-RAMs or FRAM), in which an electric field is applied in a predetermined direction between the gate and the substrate of the transistor. The electric field polarizes the gate insulation film of the transistor, thereby writing data into the memory cell. The data stored in the memory cell can be discriminated and readout by detecting the pulse current generated on reversing the polarization through application of the electric field. This readout process typically destroys the data stored in the memory cell (destructive readout), thus requiring circuits to restore it. In addition, the electric field applied to the ferroelectric in the readout process is as strong as that applied thereto in the writing process. Frequent application of such strong electric field adversely affects the lifetime of the ferroelectrics. Further, the signal current detected on readout is proportional to the area of the ferroelectric. This means that only a restricted degree of reduction can be made in the area of the cell and FE-RAM is thus unsuitable for large-scale integration.
The problems of such ferroelectric memories have long been considered with various studies as disclosed in, for example, M. L. Jeremy and M. J. Howes, A New Ferroelectric Memory Device, Metal-Ferroelectric-Semiconductor Transistor, IEEE Transactions on Electron Devices, vol. ED21, No. 8, page 499 (1974). Many other studies have been made in which ferroelectrics were used as a gate insulator of an FET and electrical resistivity of each channel of the FET is altered with electric charges induced on polarization. To read data, the electric current flowing between channels is detected. On the other hand, the remnant polarization is modified by applying an electric field to the ferroelectric to write data. BaMgF.sub.4 and Bi.sub.3 Ti.sub.4 O.sub.12 have been studied as ferroelectrics for that purpose since the ferroelectrics are restricted to those capable of being formed on a silicon or a gallium arsenide substrate. However, both ferroelectrics BaMgF.sub.4 and Bi.sub.3 Ti.sub.4 O.sub.12 have disadvantages of extremely high coercive force and low remanence. In addition, an interface layer is formed as a result of the reaction between the semiconductor and the ferroelectric, which causes degradation of characteristics and properties of the resultant memory. Further, the configuration of the thin films obtained is not matched well with the substrate, so that the end products are far from a satisfactory level of quality.
The above mentioned problem of fatigue has been recently improved. Fatigue is responsible for the relatively short lifetime of ferroelectric memories. For example, copper oxide superconductors are used as the top and bottom electrodes of the ferroelectric and these three layers are grown epitaxially into a multilayer to avoid repeated deterioration of the ferroelectric (R. Ramesh et al., "Fatigue and aging in ferroelectric PbZr.sub.0.2 Ti.sub.0.8 O.sub.3 YBa.sub.2 Cu.sub.3 O.sub.7 heterostructures," Integrated Ferroelectrics, vol. 1, No. 1, pages 1-15 (1992). This method, however, does not lead to a solution for the problem of the destructive readout. It would consequently be desirable to provide FETs that are applicable to non-volatile memories.
On the other hand, superconductive devices using oxide superconductors have been studied widely in recent years, in which the superconductive state is modulated by an electric field to provide a three terminal metal oxide-semiconductor field-effect transistor (MOSFET). Initially, Fiory et al. revealed that the carrier density of a thin YBa.sub.2 Cu.sub.3 O.sub.7 film can be modulated by an electric field ("Metallic and superconducting surfaces of YBa.sub.2 Cu.sub.3 O.sub.7 probed by electrostatic change modulation of epitaxial films", Physical Review Letters, vol. 65, No. 27 page 3441, 1990). Later, Levy et al. modulated the electric conductivity (electrical resistivity) of a YBa.sub.2 Cu.sub.3 O.sub.6 film by depositing on one surface of a SrTiO.sub.3 substrate a semiconductor-like thin YBa.sub.2 Cu.sub.3 O.sub.6 film having low carrier density and forming an Ag electrode on the other surface of the substrate, which was used as a gate electrode to apply voltage to the gate (Journal of Applied Physics, vol. 69, page 4439, 1991). Stimulated by these studies, the field effect of copper oxide superconductors has been developed and expanded around YBa.sub.2 Cu.sub.3 O.sub.7. Recently, a YBa.sub.2 Cu.sub.3 O.sub.7 ultra-thin film (film thickness of equal to or less than 50 .ANG.) has been deposited on a substrate, on which a dielectric such as SrTiO.sub.3 is deposited. The gate electrode is formed thereon to modulate the superconductive transition temperature and the critical current density of the YBa.sub.2 Cu.sub.3 O.sub.7 ultra-thin film (Applied Physics Letter, vol. 59, page 3470, 1991).
In the present invention, consideration has been given to the characteristics and properties of the copper oxides at ordinary temperatures. More particularly, this invention uses the specificity of copper oxides under the normal conductive state rather than the superconductive state when the copper oxides are applied as electronic devices. Studies have been made to modulate, by the field effect, semiconductive copper oxides having the same structure as the superconductive copper oxides, thereby providing novel FETs (Japanese Patent Application No. 203396/1992). Advanced studies have also been made with the aim of using this FET as a memory cell. In this way, the problem of the short channel effect encountered in conventional Si-MOSFETs is much improved with the thinner channel film thickness and the lower carrier concentrations suitable for the present FET. It has also been found that larger FETs of this invention can be integrated, exceeding the size limit of conventional Si-MOSFETs with less power consumption. More particularly, the limitations of Si-MOSFETs are overcome by using oxide semiconductors having a structure similar to that of the perovskite dielectrics having a large dielectric constant.
In addition, it is considered that the above mentioned ferroelectric memory is suitable for use as a nonvolatile memory in view of speed, lifetime and capability of storing information. In a preferred embodiment, the ferroelectric memory has a nondestructive readout of the FET type. However, conventional methods of modulating the electrical resistivity of the channel of Si or GaAs are disadvantageous in that it is difficult to lower the interface level due to the ferroelectric reaction and that excellent ferroelectrics can not be manufactured because of poor lattice-matching, which results in smaller degrees of electric modulations available for the electrical resistivity. The present inventor has found that the above mentioned problems can be overcome by using as the channel an oxide that has a similar lattice constant to the ferroelectric oxide and for which electrical resistivity can be varied through doping.